1. Field of the Invention
The present invention relates to a lighting circuit capable of preventing deterioration of the lighting performance of a discharge lamp and misfiring or the like of the discharge lamp by optimizing an auxiliary voltage for reignition of the discharge lamp.
2. Description of the Related Art
Recently, attention is being paid to a compact discharge lamp (a metal halide lamp or the like) as a light source for vehicular lamps. As shown in FIG. 12, for example, a known lighting circuit a is designed to include a DC power supply b, a DC power supply circuit c, and a DC-AC converter d. A DC voltage output from the DC power supply circuit c is converted by the DC-AC converter d to an AC voltage which is in turn supplied to a discharge lamp f via an inductive element e for current limitation.
To provide an auxiliary voltage for reigniting the discharge lamp f, a circuit including a diode g and a capacitor h is provided between the DC power supply circuit c and the DC-AC converter d. The diode g is so arranged to permit a current flow toward the DC-AC converter d from the DC power supply circuit c, with the capacitor h inserted between the input terminals of the DC-AC converter d at the subsequent stage of the diode g.
This lighting circuit has such a problem that while a sufficiently large auxiliary voltage for reignition is provided when a large lamp current flows at a time like the initial lighting stage of a discharge lamp, the auxiliary voltage becomes lower when a little lamp current flows as in a case where the discharge lamp is in a steady lighting state.
Specifically, since the level of the auxiliary voltage for reignition is determined by the inductance of the inductive element e, the impedance and the lamp current of the discharge lamp f and the capacitance of the capacitor h, if the auxiliary voltage is small at the time of steady lighting or the like of the discharge lamp, the ability of the lighting circuit to keep the discharge lamp on is reduced, thus impairing the lighting performance or a lighting failure of the discharge lamp may occur when the lifetime of the discharge lamp comes to an end.
Setting the capacitance of the capacitor h small to increase the auxiliary voltage at the time of steady lighting would make the auxiliary voltage at the initial lighting stage of the discharge lamp too large. This requires that the withstand voltage of the components of a peripheral circuit should be increased, resulting in a cost increase.
Accordingly, it is an object of this invention to improve the lighting performance of a discharge lamp with a simple circuit structure to thereby prevent a lighting failure at the end of the lifetime of the discharge lamp.